1. Field of the Invention
The invention relates to an electric power steering system that assists driver's steering operation.
2. Discussion of Background
There is known an electric power steering system that applies motor power to a vehicle steering mechanism so as to assist steering operation performed by a driver. Conventionally, as this type of electric power steering system, there has been a system that is described in JP2006-175940 A.
The electric power steering system described in JP2006-175940 A includes a first reference model that determines target steering torque based on a steering angle, and a second reference model that determines a target rudder angle of a steering system (a target steered angle) based on steering torque. Driving of a motor is controlled based on both of the reference models (ideal models). In other words, it is possible to constantly maintain steering torque at an optimal value by a first assist component that is obtained by performing torque feedback control so as to make actual steering torque follow target steering torque. It is also possible to cancel reverse input vibration from steered wheels by a second assist component that is obtained by performing steered angle feedback control that makes an actual steered angle follow a target steered angle.
When a vehicle is traveling, vibration is caused in steered wheels in accordance with a road condition, and the vibration is transmitted to a steering wheel through a steering mechanism. Therefore, a driver is able to grasp the road condition (road information) through the vibration felt by the hands gripping the steering wheel. However, in the electric power steering system described in JP2006-175940 A, vibration that is caused in a steering mechanism in accordance with the road condition is suppressed, and it is thus impossible for a driver to obtain a steering feel (responsive feel) corresponding to the road condition. This is one of factors that cause deterioration in the steering feel.